The present invention relates to a latex of vinylidene chloride copolymer obtained by emulsion copolymerization of vinylidene chloride and one or more monomers copolymerizable therewith using an anionic surfactant and a nonionic surfactant.
More particularly, the present invention relates to a vinylidene chloride copolymer latex: (1) which is excellent in stability when polymerized; (2) which hardly yellows with the lapse of time despite using a nonionic surfactant predominantly; and (3) which unexpectedly improves in stability when a cationic surfactant is added despite containing an anionic surfactant.
Heretofore, almost all vinylidene chloride copolymer latices have been prepared by emulsion polymerization in the presence of an anionic surfactant. It has scarcely been found that they are prepared by the use of a nonionic surfactant in emulsion polymerization. It has been found that a vinylidene chloride resin latex obtained by using such an anionic surfactant is markedly unstable when an electrolyte coexists. Consequently, for instance, in an application such as addition of a vinylidene chloride latex to a mortar, there has been adopted a countermeasure of using a nonionic surfactant together with an anionic one to prevent agglomeration due to inorganic electrolytes. However, in uses such as coating a plastic film, one will face difficulties when one wishes to use a cationic antistatic agent which has an opposite polarity to that of an anionic surfactant to acquire a high antistatic performance. From this point of view, it is known that, if a vinylidene chloride resin latex is obtained using only a nonionic surfactant, the stability thereof against an electrolyte is satisfactory when an inorganic salt or a salt of either anionic or cationic organic material is added thereto.
The vinylidene chloride resin latex obtained using a nonionic surfactant is obviously theoretically advantageous and highly desired in industries. However, such a latex can be scarcely found in the world. It is believed that this scarcity is due predominantly to the difficulty in maintaining the stability of colloid in polymerization when the emulsion copolymerization of vinylidene chloride resin is carried out using only a nonionic surfactant. In other words, there can be obtained only a product having a markedly poor mechanical stability. Secondly, even if a latex can be obtained, a film produced therefrom yellows rapidly with time. Thus, there can only be obtained a latex having a markedly low commercial value.
The present inventors started this investigation to satisfactorily solve the problem of difficulties or disadvantages which appear when a nonionic surfactant is used and to obtain a vinylidene chloride resin latex having an outstanding chemical stability. They found that the amount of surfactant used should be as small as possible in order to suppress to some extent the yellowing with time of a vinylidene chloride resin latex obtained using a nonionic surfactant. Needless to say, the stability of a latex obtained by emulsion polymerization is improved as the amount of coexisting surfactant is increased. Moreover, a latex obtained using a nonionic surfactant tends to have a mechanical stability inferior to that of a latex obtained using an anionic surfactant. Accordingly, a larger amount of nonionic surfactant is required which makes the problem more difficult. The present inventors eagerly investigated under such difficult conditions and found a fact mentioned below.
That is, it was found that the emulsion copolymerization of vinylidene chloride resin using a nonionic surfactant together with a very small amount of an anionic surfactant in accordance with a particular method shows an extremely high polymerization stability. The vinylidene chloride resin latex so obtained is markedly improved in yellowing with time which is a defect of vinylidene chloride resin latex obtained using only a nonionic surfactant. The vinylidene chloride latex so obtained shows not only an outstanding stability against the addition of an electrolyte in spite of the coexistence of an anionic surfactant but also an unexpected improvement in mechanical stability when a cationic surfactant is added. The present invention was achieved on the basis of finding this fact.